This invention relates to improvements in instruments for measuring magnetic fields, and more particularly to an improved magnetometer for use in low magnetic field strength situations characteristic of the earth's magnetic field, typically in the range of about 0.24 to about 0.64 oersted.
Magnetometers having sufficiently sensitivity to be capable of detecting or measuring small variations or anomalies in the earth's magnetic field are useful in a variety of applications including location of pipelines, cables, or other buried objects, sunken vessels, geological formations, military purposes, and the like. Among existing magnetometers for these purposes are those wherein the magnetic field sensor controls the frequency of operation of an oscillator, which frequency can be utilized to provide readout or apparatus control functions. Examples of such devices may be found in U.S. Pat. No. 2,991,414 to R. M. Tillman, U.S. Pat. No. 3,040,247 to R. L. Van Allen, and U.S. Pat. No. 3,461,387 to F. J. Morris, et al.
Magnetometers of the just mentioned type generally comprise magnetic field sensors in the form of specialized cores of high permeability material in combination with a winding forming part of the oscillator circuit. In order to achieve adequate sensitivity, the cores of the magnetometers of the above patents are subjected to an artificially high field through magnetic biasing, either by permanent magnets, by using concentrators, or by a field winding that is suitably energized. While such biasing serves to increase the permeability of the sensor core, it may introduce noise factors into the system as well as increase the weight, size and complexity of the magnetometer apparatus. Moreover, those expedients may cause undesirable effects on other instrumentation, for example in the case of magnetic mine hunting.
U.S. Pat. No. 3,239,826 to E. W. Yetter teaches the use in a transducer of a core material having a predetermined Curie point, and which changes state from paramagnetic or antiferromagnetic to ferromagnetic or ferrimagnetic at a temperature intermediate absolute zero and the Curie point, the change being characterized as a first order transition as compared to the Curie point change which is characterized as a second order transition. The exemplary core material is of relatively low permeability, on the order of 20 or 30, and would apparently require a considerable biasing field to achieve permeability that would approach the sensitivity required for use in measuring small changes in the earth's magnetic field.
Other known magnetometers include flux-gate types using second harmonic frequency generation and energy pumped gas or vapor, e.g., cesium vapor magnetometers. The former are subject to the same disadvantages mentioned earlier, while the latter is limited to total field sensing as opposed to magnetometers that are sensitive to directional components of a magnetic field and hence more useful in detecting the character of anomalies in the field. Each of these types of magnetometer requires substantial electrical power for operation, and are expensive to manufacture.